Medium voltage/high voltage/extra high voltage (MV/HV/EHV) cable construction is typically comprised one or more high potential conductors in a cable core that is surrounded by several layers of polymeric materials including a first semiconducting shield layer, an insulating layer, a second semiconducting shield layer, a metallic wire or tape shield layer used as the ground phase, and a protective jacket. Additional layers within this construction, such as a moisture barrier layer, are often included in the construction.
Polymeric semiconducting shields have been utilized in multilayered power cable construction for many decades. These shields are used to provide layers of intermediate resistivity between the high potential conductor and the primary insulation layer, and between the primary insulation layer and the ground or neutral potential layer.
The primary purpose of the semiconducting shield layer between the conductor and insulation layer within an electrical power cable construction is to ensure the long term viability of the primary solid insulation layer. The use of extruded semiconducting shields essentially eliminates partial discharge within the cable construction at the interface of conductive and dielectric, i.e., insulation, layers. Longer cable life is also realized through improvement of the conductor shield interfacial smoothness, which then minimizes any localized electrical stress concentration. Polymeric conductor shields with improved smoothness have been demonstrated to extend the cable life through accelerated testing (Burns, Eichhorn, and Reid, IEEE Electrical Insulation Magazine, Vol. 8, No. 5, 1992). HV and EHV cable applications require polymeric conductor shields with super smoothness.
Smoothness can be measured using a profilometer. For the assessment of smoothness, a statistical approach of three dimensional structures of protrusions or small lumps of semiconductive shield tapes, which are random in sizes and shapes on the surface, is used. The method determines the number of protrusions and their respective heights in semiconductive shield compounds. The heights are classified into 10 micron (μm) increments from 20-70 μm and the number of protrusions is reported as density (defects/m2). A super smooth semiconductive shield compound typically meets a specification of a maximum (max) of 200 pips/m2 with 3039 μm size; max of 20 pips/m2 with 40˜49 μm size; max 2 pips/m2 with 50˜59 μm; without pip size greater than (>) 60 μm.
One common means to achieve a smooth or super smooth interface between the semiconducting shield layer and the conductor or an insulation layer is to include acetylene carbon black in the formulation for the semiconducting shield layer. Due to the chemical and physical nature of acetylene carbon black, relative to furnace carbon black, fewer surface defects are observed on an extruded surface.
Besides carbon black selection, the process by which the semiconducting shield layer is formed also plays important role on smoothness control. The semiconductive layers are usually extruded together with the insulation layer through a triple extrusion system. Weld lines at the interface of two semiconducting shield melts can happen during a co-extrusion process and protrusions between the semiconducting shield and the insulation layers can potentially generate at the weld line position. Such protrusions can cause electrical stress concentration under high voltage and thus shorten cable life.
As such, an interest exists in a semiconducting shield composition that meets the electrical resistance and the smoothness requirements of HV and EHV cables and avoids protrusion issues in their manufacture.